Electrochemical cell with tmccc electrodes in an acetonitrile solvent including a dinitrile additive

ABSTRACT

A system and method for a liquid electrolyte used in secondary electrochemical cells having at least one electrode including a TMCCC material, the liquid electrolyte enabling an increased lifetime while allowing for fast discharge to extremely high depth of discharge. The addition of dinitriles to liquid electrolytes in electrochemical cells in which energy storage is achieved by ion intercalation in transition metal cyanide coordination compounds (TMCCC) has the advantage of increasing device lifetime by inhibiting common chemical and electrochemical degradation mechanisms.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of application Ser. No. 16/898,692filed on Jun. 11, 2020, and is related to Application PCT/US21/37102filed on Jun. 11, 2021, the contents of each of these applications arehereby expressly incorporated by reference thereto in their entiretiesfor all purposes.

FIELD OF THE INVENTION

The present invention relates generally to improvement in secondaryelectrochemical cells, and more specifically, but not exclusively, toincreasing device lifetime through inhibition of one or more degradationmechanisms in a rechargeable electrochemical device having at least oneelectrode including a transition metal cyanide coordination compound(TMCCC) material.

BACKGROUND OF THE INVENTION

The subject matter discussed in the background section should not beassumed to be prior art merely as a result of its mention in thebackground section. Similarly, a problem mentioned in the backgroundsection or associated with the subject matter of the background sectionshould not be assumed to have been previously recognized in the priorart. The subject matter in the background section merely representsdifferent approaches, which in and of themselves may also be inventions.

It is recognized in electrochemistry that it is unlikely that there willbe a single battery that works optimally for every application.Selecting the right battery for an application is about identifying themost important battery metrics and trading these off against others. Forinstance, when one desires a lot of power for an application, cellinternal resistance is often minimized, and this may be done byincreasing electrode surface area. But this also increases inactivecomponents such as current collectors and conductive aid, so energydensity could be traded off to gain power.

Important considerations may include metrics such as flexibility,safety, energy density, power density, voltage, cost, shelf life,operational life, form factor (e.g., thickness), commercialavailability, temperature range, and cycle life.

Research, development, and manufacture of various battery configurationsfocus on increasing selected subsets of these metrics which may includeparticular tuning for specific applications. Specialization of batterieshave allowed for improved batteries for a wide range of applications.

As a consequence, the components of any particular battery are tailoredfor the desired solution which often means that one component in onebattery solution may not perform similarly in another battery solution.

Discussed herein is a class of secondary electrochemical cells thatinclude a transition metal cyanide coordination compound (TMCCC)material. More specifically, this class of electrochemical cell includesliquid electrolytes in which energy storage is achieved by ionintercalation in one or more electrodes including the TMCCC material.

Performance of this class of electrochemical cell may implicate a rateof parasitic reactions during operation of the cell. Some solutions mayinclude operation within a narrower voltage window in order to diminishthe rate of parasitic reactions that result in cell degradation.However, this alternative results in a low utilization of the energystored by the cell. Operating electrochemical devices within a narrowervoltage window may result in significantly diminished energy.

Performance of this class of electrochemical cell is related to thecomposition of the liquid electrolytes. When one liquid electrolyteincludes an undesirable rate of parasitic reactions, one solution may beto substitute a different liquid electrolyte that reduces the rate ofparasitic reactions. However, that substituted liquid electrolyte mayalter performance, such as decreasing electrolyte conductivity which maylower maximum charge and discharge power. Electrolytes with an entirelydifferent solvent system, including most polar-aprotic solvents otherthan acetonitrile as the primary solvent, are disadvantageous becausetheir ionic conductivity is significantly lower than that ofelectrolytes containing acetonitrile, and therefore precludes theirapplication when high rate capability is desired.

It may be advantageous to implement a new class of liquid electrolytefor secondary electrochemical cells having at least one electrodeincluding a TMCCC material.

BRIEF SUMMARY OF THE INVENTION

Disclosed is a system and method for a liquid electrolyte used insecondary electrochemical cells having at least one electrode includinga TMCCC material, the liquid electrolyte enabling an increased lifetimewhile allowing for fast discharge to extremely high depth of discharge.The following summary of the invention is provided to facilitate anunderstanding of some of the technical features related to secondaryelectrochemical cells including at least one TMCCC electrode and is notintended to be a full description of the present invention. A fullappreciation of the various aspects of the invention can be gained bytaking the entire specification, claims, drawings, and abstract as awhole. The present invention is applicable to other configurations ofelectrochemical cells and components in addition to the examplesdiscussed and disclosed herein. For example, other electrode materialsof an electrochemical cell that may be considered “wet” that maycontribute significant quantities of water, in the context of that cellchemistry, to the cell during operation.

A class of additives for liquid electrolytes in electrochemical cellshaving an anode electrode and a cathode electrode, in which theelectrolyte salt is an alkali metal salt and at least one of the twoelectrodes contains a TMCCC material capable of intercalation anddeintercalation reactions with alkali metal cations.

Some liquid electrolytes may include solutions of an alkali metal salt,or a mixture of several different alkali metal salts, and a dinitrileadditive, in a mononitrile solvent.

An electrochemical cell including an electrolyte; an anode electrode inelectrical communication with the electrolyte; and a cathode electrodein electrical communication with the electrolyte; wherein at least onethe electrode includes a transition metal cyanide coordination compoundmaterial; and wherein the electrolyte includes a mononitrile solvent,one or more alkali metal salts in solution with the solvent, and anadditive disposed within the solvent; and wherein the additive includesa dinitrile material.

A liquid electrolyte for a secondary electrochemical cell having atleast one electrode including a transition metal cyanide coordinationmaterial, including a mononitrile solvent; and a dinitrile additivedisposed within the mononitrile solvent.

An embodiment of an electrochemical cell, including an electrolyte; ananode electrode in electrical communication with the electrolyte; and acathode electrode in electrical communication with the electrolyte;wherein the cathode electrode includes a first transition metal cyanidecoordination compound material; and wherein the electrolyte includes anorganic solvent, a salt in solution with the solvent, and an additivedisposed within the solvent; wherein the salt includes an alkali cationand a polyatomic anion including fluoride; and wherein the additiveincludes a dinitrile material.

Any of the embodiments described herein may be used alone or togetherwith one another in any combination. Inventions encompassed within thisspecification may also include embodiments that are only partiallymentioned or alluded to or are not mentioned or alluded to at all inthis brief summary or in the abstract. Although various embodiments ofthe invention may have been motivated by various deficiencies with theprior art, which may be discussed or alluded to in one or more places inthe specification, the embodiments of the invention do not necessarilyaddress any of these deficiencies. In other words, different embodimentsof the invention may address different deficiencies that may bediscussed in the specification. Some embodiments may only partiallyaddress some deficiencies or just one deficiency that may be discussedin the specification, and some embodiments may not address any of thesedeficiencies.

Other features, benefits, and advantages of the present invention willbe apparent upon a review of the present disclosure, including thespecification, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, in which like reference numerals refer toidentical or functionally-similar elements throughout the separate viewsand which are incorporated in and form a part of the specification,further illustrate the present invention and, together with the detaileddescription of the invention, serve to explain the principles of thepresent invention.

FIG. 1 illustrates cell energy versus time during float testing;

FIG. 2 illustrates cell capacity versus time during float testing;

FIG. 3 illustrates coulombic efficiency versus time during floattesting;

FIG. 4 illustrates steady state float current versus time during floattesting;

FIG. 5 illustrates 1 C constant-current charge-discharge voltageprofiles;

FIG. 6 illustrates cell energy versus time during float testing;

FIG. 7 illustrates cell capacity versus time during float testing;

FIG. 8 illustrates coulombic efficiency versus time during floattesting;

FIG. 9 illustrates steady state float current versus time during floattesting;

FIG. 10 illustrates a post-mortem SEM image;

FIG. 11 illustrates an EDX spectrum of Selected Area 1 in FIG. 10;

FIG. 12 illustrates an EDX spectrum of Selected Area 2 in FIG. 10;

FIG. 13 illustrates an FTIR of electrolyte extracted from aged cells;

FIG. 14 illustrates a magnified region of the FTIR spectra in FIG. 13;and

FIG. 15 illustrates a magnified region of the FTIR spectra in FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide a system and method for aliquid electrolyte used in secondary electrochemical cells having atleast one electrode including a TMCCC material, the liquid electrolyteenabling an increased lifetime while allowing for fast discharge toextremely high depth of discharge. The following description ispresented to enable one of ordinary skill in the art to make and use theinvention and is provided in the context of a patent application and itsrequirements.

Various modifications to the preferred embodiment and the genericprinciples and features described herein will be readily apparent tothose skilled in the art. Thus, the present invention is not intended tobe limited to the embodiment shown but is to be accorded the widestscope consistent with the principles and features described herein.

Definitions

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this general inventive conceptbelongs. It will be further understood that terms, such as those definedin commonly used dictionaries, should be interpreted as having a meaningthat is consistent with their meaning in the context of the relevant artand the present disclosure, and will not be interpreted in an idealizedor overly formal sense unless expressly so defined herein.

The following definitions apply to some of the aspects described withrespect to some embodiments of the invention. These definitions maylikewise be expanded upon herein.

As used herein, the term “or” includes “and/or” and the term “and/or”includes any and all combinations of one or more of the associatedlisted items. Expressions such as “at least one of,” when preceding alist of elements, modify the entire list of elements and do not modifythe individual elements of the list.

As used herein, the singular terms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to an object can include multiple objects unless thecontext clearly dictates otherwise.

Also, as used in the description herein and throughout the claims thatfollow, the meaning of “in” includes “in” and “on” unless the contextclearly dictates otherwise. It will be understood that when an elementis referred to as being “on” another element, it can be directly on theother element or intervening elements may be present therebetween. Incontrast, when an element is referred to as being “directly on” anotherelement, there are no intervening elements present.

As used herein, the term “set” refers to a collection of one or moreobjects. Thus, for example, a set of objects can include a single objector multiple objects. Objects of a set also can be referred to as membersof the set. Objects of a set can be the same or different. In someinstances, objects of a set can share one or more common properties.

As used herein, the term “adjacent” refers to being near or adjoining.Adjacent objects can be spaced apart from one another or can be inactual or direct contact with one another. In some instances, adjacentobjects can be coupled to one another or can be formed integrally withone another.

As used herein, the terms “connect,” “connected,” and “connecting” referto a direct attachment or link. Connected objects have no or nosubstantial intermediary object or set of objects, as the contextindicates.

As used herein, the terms “couple,” “coupled,” and “coupling” refer toan operational connection or linking. Coupled objects can be directlyconnected to one another or can be indirectly connected to one another,such as via an intermediary set of objects.

The use of the term “about” applies to all numeric values, whether ornot explicitly indicated. This term generally refers to a range ofnumbers that one of ordinary skill in the art would consider as areasonable amount of deviation to the recited numeric values (i.e.,having the equivalent function or result). For example, this term can beconstrued as including a deviation of ±10 percent of the given numericvalue provided such a deviation does not alter the end function orresult of the value. Therefore, a value of about 1% can be construed tobe a range from 0.9% to 1.1%.

As used herein, the terms “substantially” and “substantial” refer to aconsiderable degree or extent. When used in conjunction with an event orcircumstance, the terms can refer to instances in which the event orcircumstance occurs precisely as well as instances in which the event orcircumstance occurs to a close approximation, such as accounting fortypical tolerance levels or variability of the embodiments describedherein.

As used herein, the terms “optional” and “optionally” mean that thesubsequently described event or circumstance may or may not occur andthat the description includes instances where the event or circumstanceoccurs and instances in which it does not.

As used herein, the term “size” refers to a characteristic dimension ofan object. Thus, for example, a size of an object that is spherical canrefer to a diameter of the object. In the case of an object that isnon-spherical, a size of the non-spherical object can refer to adiameter of a corresponding spherical object, where the correspondingspherical object exhibits or has a particular set of derivable ormeasurable properties that are substantially the same as those of thenon-spherical object. Thus, for example, a size of a non-sphericalobject can refer to a diameter of a corresponding spherical object thatexhibits light scattering or other properties that are substantially thesame as those of the non-spherical object. Alternatively, or inconjunction, a size of a non-spherical object can refer to an average ofvarious orthogonal dimensions of the object. Thus, for example, a sizeof an object that is a spheroidal can refer to an average of a majoraxis and a minor axis of the object. When referring to a set of objectsas having a particular size, it is contemplated that the objects canhave a distribution of sizes around the particular size. Thus, as usedherein, a size of a set of objects can refer to a typical size of adistribution of sizes, such as an average size, a median size, or a peaksize.

As used herein, the term “dinitrile” means an organic chemical compoundcontaining two, but not more than two, nitrile groups. A nitrile is anyorganic compound that include a —C≡N functional group (for purposes ofthis disclosure, the prefix cyano- may be used interchangeably with theterm nitrile). For purposes of this application, a term “polynitrile”may be used to identify materials including two or more nitrile groups,with polynitrile materials including dinitrile materials while excludingmononitrile (a single nitrile group) materials. As further describedherein, the mononitrile solvent and the dinitrile additives aredesirable, purposeful, and functional and are present in sufficientquantities designed to meet design goals for the reduction of a rate ofparasitic reactions resulting from water leaving an electrode duringoperation. This is distinguished from any situation in which the nitrilegroups are viewed as undesirable, unpurposeful, and degrading such as atrace or impurity that decreases desired performance or other cellmetric.

Disclosed herein is a new class of liquid electrolytes that enableincreased lifetime of electrochemical energy storage devices, while atthe same time allowing for fast discharge to extremely high depth ofdischarge. Electrolytes containing dinitrile additives were found tohave several advantages, including: (i) in electrochemical devices thatsuffer degradation due to leaching of transition metal ions fromelectrode materials, the formation of transition metal oxideprecipitates that degrade cell performance is diminished; (ii) inelectrochemical devices that degrade due to the presence of waterimpurities, which are difficult to remove from common electrodematerials, electrolyte solvents and electrolyte salts, unwantedreactions between nitrile-containing electrolyte solvents and water aresuppressed by dinitrile additives. These effects are demonstrated hereinwith a unique sodium-ion battery in which at least one electrodematerial contains a significant concentration of water as part of itscrystal lattice, some of which is released while the device is operated.With regards to dramatically enhancing the tolerance of electrochemicalcells to water impurities, some of the examples provide a multitude ofadvantages, including significant cost savings in cell manufacturingprocesses that often require time-consuming and energy-intensive dryingprotocols and construction of controlled processing environments such asdry rooms and glove boxes around entire assembly lines, andopportunities for novel electrode materials for which full dehydrationis highly disadvantageous or unfeasible.

Literature includes a discussion of certain uses ofacetonitrile-dinitrile solvent mixtures. For example, in electrolytesfor electric double layer capacitors (EDLC), see R1, and in Li-ionbatteries, see R2-R6. Therein, benefits such as high ionic conductivity,stability of nitrile and dinitrile solvents across wide electrochemicalpotential windows, and enhanced safety against thermal runaway and firedue to the low vapor pressures of dinitriles, appear to have beendescribed. However, those discussions appear to be limited toelectrochemical structures containing ceramic or carbonaceous electrodesthat are designed to be anhydrous (while water is an undesirableimpurity, a certain quantity of water may be tolerated as an impurity),and do not show an improvement in cell stability performance as a resultof the addition of a dinitrile species to the electrolyte.

The examples provided below disclose electrochemical structures andcomponents that may afford significantly improved calendar and cyclelifetime for an electrochemical device. The examples describe novel andnon-obvious elements that are unique in design.

The examples include TMCCC electrode materials used in this device thatallow for fast ion transport and, during charge and discharge, undergoeither no phase transitions at all, or only subtle structure changes.Therefore, these examples may offer much higher rate capability and muchlonger cycle life, even when cycled repeatedly to extremely high depthof discharge.

The degradation mechanism of the devices described in the examples isdifferent from typical degradation mechanisms in Li-ion battery cells orin EDLCs, and the beneficial effect of dinitrile additives in cellscontaining TMCCC electrode material cannot be inferred from any of thebenefits claimed in prior art disclosing electrolytes containingdinitriles in Li-ion cells or EDLCs. In particular, a new method ofmitigating cell degradation mechanisms is described in which solventmolecules are decomposed within their nominal electrochemical stabilitywindow through reaction steps such as hydrolysis that do not involvesolvent reduction or oxidation.

More generally, interactions between liquid nitrile-based electrolytesand electrode materials that intercalate other ions than Li⁺, such asthe larger alkali metal ions Na⁺, K⁺, Rb⁺, or Cs⁺ have not beenaddressed. Intercalation mechanisms and ion-solvent interactions arefundamentally different between Li⁺ and larger alkali metal cations, dueto the unique small ionic radius of Li⁺ (R7).

Some examples herein include a new class of additives for liquidelectrolytes in electrochemical cells having an electrolyte including asolvent and an electrolyte salt, an anode electrode, and a cathodeelectrode, in which the electrolyte salt includes an alkali metal saltand at least one of the two electrodes includes a TMCCC material capableof intercalation and deintercalation reactions with alkali metal cationsin the electrolyte.

The liquid electrolytes may include solutions of an alkali metal salt,or a mixture of several different alkali metal salts, and a dinitrileadditive, in a mononitrile solvent. Preferred examples of alkali metalsalts include suitable salts containing an alkali metal cation and ananion, wherein the alkali metal cation is sodium, potassium, rubidium orcesium, and anions include, but are not limited to, perchlorate,tetrafluoroborate, hexafluorophosphate, difluoro-oxalatoborate,triflate, bis(trifluoromethanesulfonyl)imide, dicyanamide,tricyanomethanide, and mixtures thereof. Preferred examples of sodiumsalts include sodium salts such as, but not limited to, sodiumperchlorate, sodium tetrafluoroborate, sodium hexafluorophosphate,sodium difluoro-oxalatoborate, sodium triflate, sodiumbis(trifluoromethanesulfonyl)imide, sodium dicyanamide, and sodiumtricyanomethanide, and mixtures thereof. A preferred sodium saltincludes sodium bis(trifluoromethanesulfonyl)imide. Examples ofdinitrile additives include malonitrile, succinonitrile, glutaronitrile,and adiponitrile. Additives of note include succinonitrile andadiponitrile, with succinonitrile particularly noteworthy in some cases.Examples of mononitrile solvents include acetonitrile, propionitrile andbutyronitrile. The mass ratios of mononitrile solvent and dinitrile mayrange from approximately 99:1 to 70:30 with suitable salt concentrationsthat result in a liquid solution at a desired operating temperature. Theoperating temperature may be between −60° C. and +80° C., or a narrowerscope of temperatures within this range as needed or desired.

Other organic electrolyte solvents that are electrochemically inactivein the operating electrochemical potential range of the TMCCC electrodeand the counter electrode may be used in a practical cell. Thesesolvents include nitriles such as succinonitrile or propionitrile,carbonates including propylene carbonate or dimethyl carbonate, sulfonesincluding sulfolane and dimethyl sulfone, sulfoxides including dimethylsulfoxide, amides including dimethylformamide, ethers including glymesincluding diglyme, triglyme, tetraglyme, 1,4-dioxane, or 1,3-dioxolane,lactones including gamma-valerolactone, glycol ethers includingmethylene glycol monoethylether, or other solvents, or a combinationthereof. Other electrolyte salts that are soluble in the electrolytesolvent and that are electrochemically inactive in the operatingelectrochemical potential range of the TMCCC electrode and the counterelectrode may be used in a practical cell. These salts may includesodium hexafluorophosphate, sodium tetrafluoroborate, sodiumperchlorate, sodium (trifluoromethane)sulfonimide, sodium4,5-dicyano-2-(trifluoromethyl)imidazolide, or other sodium salts, or acombination thereof. Furthermore, as the TMCCC electrode or itscounterelectrode may undergo electrochemical reactions with othercations such as lithium, potassium, or magnesium, these salts mayinclude lithium, potassium, or magnesium salts of tetrafluoroborate,perchlorate, (fluoromethane)sulfonimide, (trifluoromethane)sulfonimide,4,5-dicyano-2-(trifluoromethyl)imidazolide, or a combination thereof.

Mass ratios with relatively high dinitrile content result in a greaterenhancement of cell lifetime than those with lower dinitrile content asshown for adiponitrile and succinonitrile in Examples 1 and 2,respectively, whereas initial electrolyte conductivity increases withacetonitrile content, due to the low viscosity of acetonitrile.Preferred mass ratios of acetonitrile and dinitrile are in the rangefrom 99:1 to 75:25. Particularly preferred mass ratios are between 95:5and 80:20 for some of the applications described herein.

One mechanism by which dinitrile additives may enhance cell life most ischelation of dissolved transition metal ions by the individual dinitrilemolecules. Side chemical reactions between the electrodes andelectrolyte that result in dissolved transition metal ions are commonlyobserved with cathode materials in lithium-ion and sodium-ion batteries,especially with cathode materials containing manganese cations. Inaddition to cathode materials that are made of mixed transition metaloxides, electrode materials consisting of transition metal cyanidecoordination compound (TMCCC) materials are also subject to partialtransition metal dissolution reactions. Most commonly, TMCCC materialswill react with the electrolyte to release manganese or iron cationsinto the electrolyte. The presence of a dinitrile electrolyte additivemay result in a formation of a chemically stable chelation complex ofone or more dinitrile molecules with the dissolved transition metalions, which may decrease a reactivity of these ions towards other cellcomponents.

Another possible advantage of electrolytes having compositions thatinclude one or more dinitrile additives may be an enhanced tolerance ofthe electrochemical device towards water impurities. Degradationmechanisms that involve unwanted reactions with water can be exacerbatedin battery cells that use TMCCC electrodes, because TMCCC electrodestypically contain significant amounts of intercalated and/or coordinatedwater molecules, potentially a significant quantity of which may bereleased during operation of the electrochemical cell including suchelectrodes. The release of these water molecules from the TMCCCelectrodes may result in significant concentrations of water beingpresent in the electrolyte of thousands to tens of thousands of partsper million, even in an event when the electrochemical cell is initiallyconstructed with anhydrous liquid electrolyte prior to operation.

FIG. 1 illustrates cell energy versus time during float testing at 1.86Vwith daily 1 hour discharge to 1.19V at an ambient temperature of 40°C., normalized to the discharge energy of the first tested cycle, forcells containing a sodium manganese iron hexacyanoferrate cathode, asodium manganese hexacyanomanganate anode, and acetonitrile-basedelectrolyte with no additive (Control) and various concentrations ofadiponitrile.

FIG. 2 illustrates cell capacity versus time during float testing at1.86V with daily 1 hour discharge to 1.19V at an ambient temperature of40° C., normalized to the discharge capacity of the first tested cycle,for cells containing a sodium manganese iron hexacyanoferrate cathode, asodium manganese hexacyanomanganate anode, and acetonitrile-basedelectrolyte with no additive (Control) and various concentrations ofadiponitrile.

FIG. 3 illustrates coulombic efficiency versus time during float testingat 1.86V with daily 1 hour discharge to 1.19V at an ambient temperatureof 40° C., for cells containing a sodium manganese iron hexacyanoferratecathode, a sodium manganese hexacyanomanganate anode, andacetonitrile-based electrolyte with no additive (Control) and variousconcentrations of adiponitrile.

FIG. 4 illustrates steady state float current versus time during floattesting at 1.86V with daily 1 hour discharge to 1.19V at an ambienttemperature of 40° C., for cells containing a sodium manganese ironhexacyanoferrate cathode, a sodium manganese hexacyanomanganate anode,and acetonitrile-based electrolyte with no additive (Control) andvarious concentrations of adiponitrile.

FIG. 5 illustrates 1 C constant-current charge-discharge voltageprofiles of cells containing a sodium manganese iron hexacyanoferratecathode, a sodium manganese hexacyanomanganate anode, andacetonitrile-based electrolyte with or without (Control) a 20%succinonitrile additive.

FIG. 6 illustrates cell energy versus time during float testing at 1.81Vwith daily 1 hour discharge to 1.19V at an ambient temperature of 55°C., normalized to the discharge energy of the first tested cycle, forcells containing a sodium manganese iron hexacyanoferrate cathode, asodium manganese hexacyanomanganate anode, and acetonitrile-basedelectrolyte with no additive (Control) and various concentrations ofsuccinonitrile.

FIG. 7 illustrates cell capacity versus time during float testing at1.81V with daily 1 hour discharge to 1.19V at an ambient temperature of55° C., normalized to the discharge capacity of the first tested cycle,for cells containing a sodium manganese iron hexacyanoferrate cathode, asodium manganese hexacyanomanganate anode, and acetonitrile-basedelectrolyte with no additive (Control) and various concentrations ofsuccinonitrile.

FIG. 8 illustrates coulombic efficiency versus time during float testingat 1.81V with daily 1 hour discharge to 1.19V at an ambient temperatureof 55° C., for cells containing a sodium manganese iron hexacyanoferratecathode, a sodium manganese hexacyanomanganate anode, andacetonitrile-based electrolyte with no additive (Control) and variousconcentrations of succinonitrile.

FIG. 9 illustrates steady state float current versus time during floattesting at 1.81V with daily 1 hour discharge to 1.19V at an ambienttemperature of 55° C., for cells containing a sodium manganese ironhexacyanoferrate cathode, a sodium manganese hexacyanomanganate anode,and acetonitrile-based electrolyte with no additive (Control) andvarious concentrations of succinonitrile.

FIG. 10 illustrates post-mortem SEM image of a sodium manganese ironhexacyanoferrate cathode taken from a cell containing anacetonitrile-based electrolyte with no additives after 340 days of floattesting at 45° C. at 1.86 V. Selected Area 1 and Selected Area 2 markthe regions in which EDX spectra in FIGS. 11 and 12 were recorded,respectively.

FIG. 11 illustrates EDX spectrum of Selected Area 1 in FIG. 10, showingthe elemental composition characteristic of sodium manganese ironhexacyanoferrate cathode active material with binder and conductivecarbon, and sodium bis(trifluoromethanesulfonyl)imide.

FIG. 12 illustrates EDX spectrum of Selected Area 2 in FIG. 10, showingthe elemental composition characteristic of a sodium manganese ironhexacyanoferrate cathode covered with manganese oxide precipitates, andsodium bis(trifluoromethanesulfonyl)imide.

FIG. 13 illustrates FTIR of electrolyte extracted from aged cellscontaining a sodium manganese iron hexacyanoferrate cathode and a sodiummanganese hexacyanomanganate anode. One cell was filled with 0.88 MNaTFSI electrolyte made with 100% acetonitrile, the other cell withelectrolyte containing 0.88 M NaTFSI in acetonitrile and 29.5% by weightsuccinonitrile. Both cells had undergone constant-voltage float testingat 1.86 V with daily 1 C constant-current discharge to 1.19 V at anambient temperature of 45° C. for a duration of 340 days.

FIG. 14 illustrates magnified region of the FTIR spectra in FIG. 13, andFIG. 15 illustrates magnified region of the FTIR spectra in FIG. 13.

EXAMPLE 1

Testing of sodium-ion battery cells containing a sodium manganese ironhexacyanoferrate cathode, a sodium manganese hexacyanomanganate anode,and mononitrile-based electrolyte containing adiponitrile additive.

Electrolytes were prepared by mixing acetonitrile with adiponitrile(ADN) to obtain two different adiponitrile concentrations of 8.5% and29.15% by weight. NaTFSI salt was dissolved in the liquid mixtures to aconcentration of 0.88 mol/L. Two groups of otherwise identical pouchcells were filled with the 8.5% ADN and the 29.15% ADN electrolyte,respectively. A third group was filled with a 0.88 mol/L NaTFSI/100%acetonitrile solution to serve as a control group. Anodes and cathodeswere carefully selected to minimize differences in mass loadings betweencells, and to originate from the same respective material synthesis andelectrode coating runs. All cells were subjected to an accelerated agingtest, in which the cells were held at a constant ambient temperature of40° C. and underwent a cycling schedule consisting of constant-currentcharging at 1 C to a cell voltage of 1.86 V, followed by a 22 hconstant-voltage period and then a constant-current discharge at 1 C to1.19 V. All cells exhibited approximately linear capacity fade over atest duration of 20 days. The cell degradation was slowed down by theaddition of adiponitrile, and a monotonic decrease of fade rates withincreasing adiponitrile concentration was observed. (FIG. 1 and FIG. 2).Significant improvements of the coulombic efficiency were also achieved.The coulombic efficiency increased monotonically with adiponitrileconcentration, from 97% with no additive to 97.8% and 98.5% with 8.5%and 29.15% adiponitrile, respectively (FIG. 3). FIG. 4 illustrates asteady-state float current averaged over the last 2 h of each floatsequence, which can be seen as a measure of the parasitic reaction rate.The adiponitrile additive at 8.5% and 29.15% reduces the steady-statefloat current by approximately 25% and 50%, respectively.

EXAMPLE 2

Testing of sodium-ion battery cells containing a sodium manganese ironhexacyanoferrate cathode, a sodium manganese hexacyanomanganate anode,and mononitrile-based electrolyte containing succinonitrile additive.

Electrolytes were prepared by mixing acetonitrile with succinonitrile(SN) to obtain two different succinonitrile contents of 10%, 15%, and20% by weight. NaTFSI salt was dissolved in the liquid mixtures to givea salt concentration of 0.88 mol/L. Three groups of otherwise identicalpouch cells were filled with the 10% SN, 15% SN, and 20% SN electrolyte,respectively. A fourth group was filled with a 0.88 mol/L NaTFSI/100%acetonitrile solution to serve as a control group. Anodes and cathodeswere carefully selected to minimize differences in mass loadings betweencells, and to originate from the same respective material synthesis andelectrode coating runs. The cells exhibited reversible voltage profilesin 1C constant current charge-discharge cycling, and no significantchanges to the characteristic features of the voltage profiles werefound between cells with or without a dinitrile additive (FIG. 5). Allcells were subjected to an accelerated aging test, in which the cellswere held at an ambient temperature of 55° C. and underwent a cyclingschedule consisting of constant-current charging at 1C to a cell voltageof 1.81 V, followed by a 22 h constant-voltage period and then aconstant-current discharge at 1 C to 1.19 V. The cell degradation wassignificantly diminished by the addition of succinonitrile, as indicatedby the energy and capacity fade (FIG. 6 and FIG. 7), in which cells withsuccinonitrile additive become clearly distinguishable from the controlsafter 30 days. The addition of succinonitrile was found to be highlyeffective at suppressing parasitic reactions, as evident in increasedcoulombic efficiencies and reduced float currents (FIG. 8 and FIG. 9).In the control cells, low coulombic efficiencies, and high floatcurrents during the initial 20 days of testing indicate high parasiticreaction rates. These parasitic reactions eventually diminish,presumably because water released from at least one of the electrodematerials is consumed, but a second parasitic mechanism becomes apparentas the steady-state float current increases again, which coincides withaccelerated capacity fade. The succinonitrile additive improves thecoulombic efficiency and diminishes the float current, and a monotonicdecrease of the float current and a monotonic increase of the coulombicefficiency with succinonitrile concentration are observed. Furthermore,the succinonitrile additive prevents the second stage of celldegradation. After 30 days, the parasitic currents remain at a low levelin cells with succinonitrile. In contrast, the cells without additivesuffer a steady increase of parasitic current beginning at 25 days, andafter 60 days their parasitic currents are 2× as high as in cells withsuccinonitrile.

EXAMPLE 3

Post-mortem analysis of a sodium-ion battery cell containing a sodiummanganese iron hexacyanoferrate cathode and a sodium manganesehexacyanomanganate anode using SEM-EDX.

A post-mortem analysis was performed on one cell made with 29.5%succinonitrile and one cell made without additives. Both cells hadundergone constant-voltage float testing at 1.86 V with daily 1 Cconstant-current discharge to 1.19 V for 340 days. The cells werebrought to a voltage of 1.56 V and disassembled under dry nitrogenatmosphere.

This post-mortem analysis included use of scanning electron microscopyimaging (SEM) and energy-dispersive x-ray spectroscopy (EDX) on TMCCCelectrodes after operation in electrochemical cells. The composition ofthese TMCCC electrodes included manganese cations. Characterizationusing the SEM and EDX techniques provide evidence that manganese oxideprecipitates had formed on a TMCCC cathode that was operated in anelectrolyte that did not contain dinitrile additives. In another cell,which was made using the identical design except for the electrolyte,which contained a succinonitrile additive in a 70.54:29.46 mass ratiobetween acetonitrile and succinonitrile (the dinitrile additive), nomanganese oxide precipitates were found with SEM-EDX. Thus, the additionof the dinitrile electrolyte additive suppressed the release ofmanganese cations from the TMCCC electrode and formation of a manganeseoxide impurity phase. This suppression is important because theformation of that manganese oxide phase resulted in an increase in theinternal resistance of the cell and a decrease in the cell's capacityand energy.

SEM images of the control cathode reveal regions with brighter anddarker contrast having otherwise similar surface morphology (FIG. 10).EDX spectra were taken in regions with bright and dark contrast,respectively. EDX found a difference in elemental composition betweenthe brighter and darker. An EDX spectrum taken in a bright region (FIG.11) shows nearly the same composition as spectra of pristine cathodesamples with relative intensities of C, N, O, Na, Mn and Fe fluorescencelines reflecting the composition of the cathode active material andadditional contributions from conductive carbon additive and binder tothe C fluorescence line. Sulfur and fluorine emission lines are alsopresent since some electrolyte salt remains within the electrodes duringsample preparation. A spectrum taken in a region that appears darker inimaging is markedly different (FIG. 12); here, the Fe fluorescence isstrongly diminished and the intensities from O and Mn are stronglyenhanced. This indicates that in the darker regions the cathode activematerial is covered with manganese oxide precipitates.

In the SEM-EDX analysis of cathode samples from the cell with 29.5%succinonitrile, no manganese oxide precipitates were detected. Thisfinding supports the hypothesis that dinitrile additives such assuccinonitrile suppress the release of manganese cations from the TMCCCelectrodes and the formation of metal oxide precipitates.

EXAMPLE 4

ATR-FTIR analysis of electrolyte extracted from a sodium-ion batterycell containing a sodium manganese iron hexacyanoferrate cathode and asodium manganese hexacyanomanganate anode.

Electrolyte was extracted from each cell in Example 3 using thefollowing procedure: a 3 cm×2 cm section of each electrode was enclosedin a centrifuge vial with an added amount of 0.1 mL acetonitrile. Afterallowing the added acetonitrile to soak into the electrode, the vial wascentrifuged at 6,500 rpm for 20 minutes. ATR-FTIR spectra of the thusobtained liquid extracts were measured using a single-reflection diamondprism.

ATR-FTIR spectra of electrolyte extracted from the aged cells with 0%and 29.5% succinonitrile are illustrated in FIG. 13. All absorptionbands in the pristine electrolyte can be assigned to vibrational modesof acetonitrile and of the bis(trifluoromethanesulfonyl)imide anion.Trace amounts of water are also present, as indicated by broadabsorption bands at wavenumbers greater than 3400 cm−1 (FIG. 14).

The spectrum of the electrolyte extracted from the aged cell in thecontrol group shows additional bands characteristic of the N—H stretch(3480 cm−1 and 3380 cm−1) (FIG. 13 and FIGS. 14) and C═O stretch modes(1680 cm−1) in acetamide (FIG. 13 and FIG. 15). In the spectrum ofelectrolyte extracted from the cell with 29.5% succinonitrile, the amideabsorption bands are also present but with strongly diminished intensityof approximately 30% compared to the control sample. The C═O stretchmode appears at slightly higher wavenumber (1690 cm−1), which could beexplained by the presence of succinonitrile.

The ATR-FTIR spectra reveal the presence of an acetamide impurity inboth cells, and that the concentration of acetamide is strongly reducedin the presence of the succinonitrile additive. In a cell containingelectrolyte comprised of acetonitrile and succinonitrile in a mass ratioof 70.54:29.46, characterization using ATR-FTIR showed a concentrationof acetamide three times lower than in other cells containing anelectrolyte containing acetonitrile and no succinonitrile. All cells inthis example were otherwise identical, except for the addition of adinitrile electrolyte additive. No acetamide was initially present ineither of these cells. Acetamide is a common reaction product ofacetonitrile with water, indicating that water impurities in the cellreacted with the electrolyte solvent during cell operation. This findinghas important implications regarding the stability of acetonitrile orother mononitrile electrolytes in electrochemical energy storagedevices. A common method of determining stability of a new electrolyterelies on determining its electrochemical stability window; this istypically done by measuring cyclic voltammetry curves of the electrolytein a three-electrode cell with a chemically inert working electrode suchas platinum metal or glassy carbon. The stability window is then takenas the range of electrochemical potentials within which the cyclicvoltammetry plot exhibits only small charge-discharge currentsassociated with the electric double layer capacitance of the workingelectrode. Using this method, mononitriles were reported to be amongthose solvents that have the widest electrochemical stability windows.However, this characterization fails to consider side reactions that donot involve oxidation or reduction of nitriles. Our findings in Example4 show that water, which is the most ubiquitous impurity in anhydrouselectrochemical devices, including Li-ion batteries and EDLCs, iscapable of decomposing acetonitrile in a reaction that is independent ofdevice voltage. Amides have significantly narrower electrochemicalstability windows than their parent nitriles. Therefore, even when wateris present only at trace levels, it can set off a chain of parasiticreactions in which amide, or amide oxidation and amide reductionproducts can continue to activate nitrile solvent molecules. Dinitrileelectrolyte additives break this runaway reaction chain at an earlystage by suppressing the hydrolysis reaction. By preventingdecomposition of the mononitrile electrolyte solvent, the dinitrileadditive extends the life of the cell containing trace water impurities.

References: (The following references are discussed herein and arehereby expressly incorporated their entireties by reference thereto.)

-   -   R1: U.S. Pat. No. 5,418,682.    -   R2: U.S. Pat. No. 9,666,906.    -   R3: WO 2008/138132 A1;    -   R4: Q. Zhang et al., “Safety-Reinforced Succinonitrile-Based        Electrolyte with Interfacial Stability for High-Performance        Lithium Batteries” Appl. Mater. Interfaces 2017, 9 (35), 29820;    -   R5: H. Duncan et al., “Electrolyte Formulations Based on        Dinitrile Solvents for High Voltage Li-Ion Batteries” J.        Electrochem. Soc. 2013, 160 (6), A838;    -   R6: A. Abouimrane et al., “Investigation of Li salt doped        succinonitrile as potential solid electrolytes for lithium        batteries” J. Power Sources 2007, 174, 883; and    -   R7: T. A. Pham et al., “Solvation and Dynamics of Sodium and        Potassium in Ethylene Carbonate from Ab Initio Molecular        Dynamics Simulations” J. Phys. Chem. C 2017, 121 (40), 21913.

The systems and methods above have been described in general terms as anaid to understanding details of preferred embodiments of the presentinvention. In the description herein, numerous specific details areprovided, such as examples of components and/or methods, to provide athorough understanding of embodiments of the present invention. Somefeatures and benefits of the present invention are realized in suchmodes and are not required in every case. One skilled in the relevantart will recognize, however, that an embodiment of the invention can bepracticed without one or more of the specific details, or with otherapparatus, systems, assemblies, methods, components, materials, parts,and/or the like. In other instances, well-known structures, materials,or operations are not specifically shown or described in detail to avoidobscuring aspects of embodiments of the present invention.

Reference throughout this specification to “one embodiment”, “anembodiment”, or “a specific embodiment” means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention and notnecessarily in all embodiments. Thus, respective appearances of thephrases “in one embodiment”, “in an embodiment”, or “in a specificembodiment” in various places throughout this specification are notnecessarily referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics of any specificembodiment of the present invention may be combined in any suitablemanner with one or more other embodiments. It is to be understood thatother variations and modifications of the embodiments of the presentinvention described and illustrated herein are possible in light of theteachings herein and are to be considered as part of the spirit andscope of the present invention.

It will also be appreciated that one or more of the elements depicted inthe drawings/figures can also be implemented in a more separated orintegrated manner, or even removed or rendered as inoperable in certaincases, as is useful in accordance with a particular application.

Additionally, any signal arrows in the drawings/Figures should beconsidered only as exemplary, and not limiting, unless otherwisespecifically noted. Combinations of components or steps will also beconsidered as being noted, where terminology is foreseen as renderingthe ability to separate or combine is unclear.

The foregoing description of illustrated embodiments of the presentinvention, including what is described in the Abstract, is not intendedto be exhaustive or to limit the invention to the precise formsdisclosed herein. While specific embodiments of, and examples for, theinvention are described herein for illustrative purposes only, variousequivalent modifications are possible within the spirit and scope of thepresent invention, as those skilled in the relevant art will recognizeand appreciate. As indicated, these modifications may be made to thepresent invention in light of the foregoing description of illustratedembodiments of the present invention and are to be included within thespirit and scope of the present invention.

Thus, while the present invention has been described herein withreference to particular embodiments thereof, a latitude of modification,various changes and substitutions are intended in the foregoingdisclosures, and it will be appreciated that in some instances somefeatures of embodiments of the invention will be employed without acorresponding use of other features without departing from the scope andspirit of the invention as set forth. Therefore, many modifications maybe made to adapt a particular situation or material to the essentialscope and spirit of the present invention. It is intended that theinvention not be limited to the particular terms used in followingclaims and/or to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include any and all embodiments and equivalents falling within thescope of the appended claims. Thus, the scope of the invention is to bedetermined solely by the appended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. An electrochemical cell, comprising: anelectrolyte; an anode electrode in electrical communication with saidelectrolyte; and a cathode electrode in electrical communication withsaid electrolyte; wherein said cathode electrode includes a firsttransition metal cyanide coordination compound material; and whereinsaid electrolyte includes an organic solvent, a salt in solution withsaid solvent, and an additive disposed within said solvent; wherein saidsalt includes an alkali cation and a polyatomic anion includingfluoride; and wherein said additive includes a dinitrile material. 2.The electrochemical cell of claim 1 wherein each said transition metalcyanide coordination compound material includes a compositionA_(x)P_(y)[R(CN)₍₆₎]_(z) n(H₂O), wherein A includes an alkali metalcation, wherein a P includes a transition metal cation, wherein Rincludes a transition metal cation, and wherein 0≤x≤2, y=1, 0.75≤z≤1,and 0≤n≤6.
 3. The electrochemical cell of claim 2 wherein P of saidcathode electrode includes at least one of manganese and iron; andwherein R of said cathode electrode includes at least one of manganeseand iron.
 4. The electrochemical cell of claim 1 in which said dinitrilematerial includes a linear dinitrile material.
 5. The electrochemicalcell of claim 4 in which said linear dinitrile material includessuccinonitrile.
 6. The electrochemical cell of claim 4 in which saidlinear dinitrile material includes adiponitrile.
 7. The electrochemicalcell of claim 1 having a weight ratio N of said solvent to said additivewherein 70:30<N<99:1.
 8. The electrochemical cell of claim 2 wherein Aincludes one or more of Li, Na, K, Rb, and Cs cations.
 9. Theelectrochemical cell of claim 1 in which said polyatomic anion includesa fluoridated sulfonimide.